A large number of disposable biomedical electrodes for heartbeat monitoring and the like are currently available. Such electrodes are designed to detect variations in the electrical potentials which appear on the skin of a patient and which reflect heartbeat activity or other electrophysiological activity. Since these skin potentials are very small--on the order of 2 millivolts--the potentials must be amplified to a considerable extent by the testing apparatus to provide effective outputs reflecting the electrophysiological activity. For this reason, electrodes must have very high performance to minimize noise factors and maximize the quality of the signals transmitted to the testing apparatus by the electrodes.
Conventional disposable ECG electrodes generally comprise an electrically conductive sensing element, preferably metal, having a substantially flat base portion or flange and a vertically-projecting pin or knob on the upper surface of the flange. The pin is either connected directly via a lead wire to the testing apparatus (one-piece connector), or it is inserted into a hollow snap connector which is, in turn, connected to the lead wire. This "male/female" snap connector arrangement is often preferred because it provides means for mechanically securing a flexible adhesive-coated material between the upper surface of the flange and base of the snap connector. The adhesive-coated material secures the electrode to the skin. A conformable, electrically-conductive interfacing material is typically used between the lower surface of the flange-portion of the sensing element and the skin to enhance electrical conductivity. The skin-interfacing material most frequently used is an electrolyte gel containing dissolved ions. Many disposable electrodes are pre-gelled during manufacture, generally by attaching a porous sponge saturated with the gel to the lower surface of the sensing element.
The majority of conventional disposable ECG electrodes are termed "silver/silver chloride" electrodes. These electrodes contain a silver or silver-coated sensing element having a layer of silver chloride deposited on the surface of the silver. It is well known that these electrodes are highly "reversible" electrochemically. When used with an electrolyte gel containing dissolved chloride ions, the electrode is able to recover rapidly after a high voltage overload, such as occurs when a patient is defibrillated. Defibrillation overload recovery is important so that the physician can obtain immediate feedback on the state of the patient's heart.
The formation of silver chloride on the silver sensing element has become as much an art as a science as can be gathered from the published literature such as the classical work of Ives and Janz in "Reference Electrodes", 1961, Academic Press, pages 179 to 226.
Electrochemically depositing a layer of silver chloride on the silver sensing element is perhaps the most common method employed. One major disadvantage of this method is that the entire surface of the silver substrate is coated with silver chloride, even though only the lower surface of the flange (which is the only part of the sensing element in contact with the electrolyte gel) requires chloriding. The result is that, in an electrode having a one-piece connector, the lead wire from the test apparatus contacts a silver chloride element. This may result in inferior electrical contact because the electrical resistivity of silver chloride is much greater than that of metal, particularly silver. A further complication associated with chloriding the entire surface of the sensing elements is that, in electrodes having a two-piece snap connector, the metal snap connector reacts chemically with the silver chloride on the sensing element to which it is anchored and undergoes oxidation. This is a common and serious problem associated with silver/silver chloride electrodes. Such reactions between the silver chloride on the sensing element and the metal snap will induce spurious signals, i.e., electrical noise or artifact in the normal ECG. To solve this problem, many electrode manufacturers have chosen to silver plate the metal snap connector. This greatly increases the cost of the electrode, and is therefore not a desirable solution.
Several alternatives have been suggested to avoid the need to deposit silver chloride electrochemically over the entire surface of the sensing element. British Pat. No. 1,350,368 describes a sensing element made by roll-bonding a thin layer of silver chloride material to a silver substrate. The manufacturing process undoubtedly involves the use of special machines and is a multistep procedure which is a disadvantage, particularly inasmuch as the silver chloride and the silver are in a fragile form during assembly.
U.S. Pat. No. 4,114,263 describes a method wherein the portion of the sensing element that is exposed to the electrolyte gel can be selectively chlorided by the passage of a D.C. electrical charge. This method is not desirable from a manufacturing standpoint since it requires that a high quantity of electrical charge be passed in a very short time interval. Furthermore, a high chloride-containing electrolyte in contact with the silver sensing element is necessary.
An additional problem often encountered with pre-gelled disposable silver/silver chloride electrodes is the gradual decomposition of silver chloride to metallic silver and chloride ions. Decomposition may result from a number of factors including leakage of the electrolyte gel from the gel chamber to the metal snap connector, and chemical impurities in the silver chloride and/or the gel. Such factors cause local galvanic cells to form which reduce silver chloride to silver metal. As the silver chloride decomposes, the electrode loses its ability to recover after defibrillation overload.
Accordingly, prior to the present invention, the need existed for a low-cost disposable ECG electrode having the ability to recover after defibrillation overload. In particular, the need existed for a disposable silver/silver chloride electrode in which the silver chloride was deposited only on the portion of the sensing element in contact with the electrolyte gel and decomposition of the silver chloride with age was eliminated.
The present invention effectively fulfills the aforementioned need by providing an electrode in which silver chloride (or like material necessary to provide recovery after defibrillation overload) is deposited on the portion of the sensing element contacting the electrically-conductive interface material, e.g., gel, continuously during the life of the electrode. This is accomplished chemically by incorporating the necessary chemical agents into the skin-interfacing material.
It is known that an electrode comprising a solid silver sensing element in contact with an electrolyte gel of high (e.g., 2.5%) chloride ion concentration will have some silver chloride formed on the sensing element. Although claimed as a silver/silver chloride electrode, the quantity of silver chloride formed on the surface of the solid silver sensing element is vanishingly small. The reaction of the sodium chloride in the gel electrolyte forms only an infinitesimal amount of silver chloride on the silver surface. This amount of silver chloride under certain circumstances of use, such as during a defibrillation procedure, is consumed by the electrical charge passed through the electrode. Hence, the electrode is polarized to undesirably high voltages which leads to long ECG trace recovery times. Furthermore, the use of the high chloride content in the gel tends to cause skin irritation.
The present invention involves the incorporation of an oxidizing agent into the electrically-conductive interface material to facilitate the deposition of silver chloride or functionally equivalent material on the sensing element. It is known that an oxidizing agent added to a solution containing chloride ions and metallic silver will result in the production of silver chloride. Such a technique has been used to form a silver halide layer on reference electrodes used for ion-selection membrane electrodes (Research Disclosure, No. 19023, February, 1980 and Research Disclosure, No. 18789, November, 1979). However, these electrodes are not subjected to the considerable quantities of electrical charge to which a conventional pregelled disposable electrode is exposed, such as during a defibrillation procedure. Prior to the present invention, oxidizing agents have not been incorporated into electrically conductive skin-interfacing materials such as electrolyte gels used in biomedical electrodes to provide continuous deposition of silver chloride or like material on the sensing element during the life of the electrode.